Process of washing and protecting a diffusion transfer photographic silver print



United States Patent 3,533,789 PROCESS OF WASHING AND PROTECTING A DIFFUSION TRANSFER PHOTOGRAPHIC SILVER PRINT Edwin H. Land, Cambridge, and Meroe M. Morse, Boston, Mass., assignors to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware No Drawing. Filed Sept. 15, 1965, Ser. No. 487,578 Int. Cl. G03c 5/54, 5/26 U.S. Cl. 96-29 12 Claims ABSTRACT OF THE DISCLOSURE The stability of silver images produced by diffusion transfer photographic techniques is increased by applying to the silver print an aqueous solution having a pH of at least 8 and comprising a polymeric material which possesses acid functional groups and forms a protective coating upon drying. In addition to the acidic film-forming polymeric material, the solution contains a volatile base in an amount sufiicient to establish in the solution a pH of 8 or higher and additionally may contain a watermiscible organic solvent for the polymeric material and a salt of a heavy metal capable of forming an insoluble sulfide.

This invention relates to the protecting of photographic images by forming protective coatings therefor, to the washing and protecting of photographic silver images by removing residual processing reagents therefrom and forming protecting coatings thereon, and to the photographic products of such processes.

Objects of the present invention are: to provide a process for increasing the stability of a photographic silver image with a novel composition comprising water for washing the image when the composition is applied and a dispersion of such materials for forming a coating upon the image when the composition is dried; to provide, as a novel product, a protected photographic image produced by such a process; and to provide for use in such a process of the foregoing type, a composition comprising a mixture of an acid polymer and a volatile base.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others, and the composition and product possessing the features, properties and relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the appended claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description.

The process of the present invention is particularly useful for washing and protecting thin photographic silver images of the type prepared by well-known diffusion transfer processes. Thin photographic silver images of this type ordinarily retain traces of the photographic reagents with which they have been processed and the continued presence of which may adversely afl ect their stability. For example, silver may be oxidized by sulfur from the residue of sodium thiosulfate which, in some cases, is employed as a solvent. Or, the silver may be oxidized by numerous oxidizing agents present in the atmosphere. Also, the image may comprise traces of silver ion in equilibrium with the metallic silver; where the image is subjected to materials which form insoluble silver residues, such as sulfides, they react with silver ion, causing more metallic silver to shift to the ionic state, ultimately destroying the silver image entirely. Furthermore, traces of unoxidized developing agent, if oxidized by atmospheric oxygen, may discolor the highlights of the image.

It has been proposed, in order to improve the stability of such an image, to coat it with a composition comprising an aqueous solution of a film-forming material. The water acts to wash traces of the photographic reagent from the image and to so distribute the film-forming material that it provides an image with a protective coating when the dispersion is dried. Processes of this type are more fully described in U.S. Pat. No. 2,719,791 issued to Edwin H. Land on Oct. 4, 1955.

In accordance with the present invention, a preferred composition for this purpose comprises a mixture of compatible components which together possess properties not possessed by the components themselves. These components include a film-forming polymeric material having acid functional groups thereon, and a volatile alkaline material such as ammonia. Preferably, the composition also comprises water, a water-miscible organic solvent and a salt of a heavy metal which forms an insoluble sulfide. Such a composition, when used as a protective coating for photographic silver images of the type described, has been found to provide the following unusual combination with respect to the images: substantial impermeability to water, elemental sulfur, and hydrogen sulfide, for example, from atmospheric or other sulfurcontaining materials with which the composition may come into contact; image stability" throughout wide temperature and humidity ranges; resistance to generalized oxidation of silver; resistance to the effects of prolonged solar radiation; and optical clarity.

U.S. Pat. No. 2,830,900, issued Apr. 15, 1958 to Edwin H. Land, Elkan R. Blout, and Howard C. Haas, discloses the washing and protecting of photographic silver images with an acid film-forming material. In this last-mentioned patent, as well as in other coating compositions of the nature described which are known to the art, an acidic or neutral environment has been established, this having been considered necessary in view of the fact that any traces of developing solution being removed are invariably alkaline in nature. Although in some instances, alkaline materials have been employed in the preparation of the coating composition, such as in U.S. Pat. No. 2,956,877, issued Oct. 18, 1960 to Edwin H. Land, Elkan R. Blout, and Howard C. Haas, the function of the base was solely to dissolve acidic polymers; the final composition was neutral or acidic in character. However, it has been found that with some classes of developing agents, the coating compositions employed heretofore are inadequate, in that they tend to blur or bleach the photographic image on the element being coated.

In accordance with the present invention, it has been found that the incorporation of a volatile alkaline material, preferably ammonia, in fairly substantial quantity in the coating composition provides a material having unexpectedly good stabilizing qualities and does not tend to distort or bleach developed photographic silver images. Although the precise quantity of alkali is not critical, where the developing agent used is acid sensitive it has generally been found that compositions having a pH of at least about 8 and preferably 9 or higher provide optimum results. Further, although ammonia is the preferred alkaline material owing to its high degree of volatility, other bases which are volatile and which have little or no crystalline or opaque residue which might obscure or disfigure the image also provide satisfactory ingredients, such as ethylamine, diethylamine, triethylamine, and butylamine.

It is desirable that the coating solution contain an agent capable of becoming dispersed in the protective coating and of protecting the image by reacting with atmospheric sulfides that may penetrate the protective coating. Preferred protective agents or salts, preferably water-soluble, are those containing heavy metal cations which form insoluble sulfides. These salts, for example, are composed of: cations such as zinc, cadmium, zirconium, and tin; and anions such as acetate, sulfate, nitrate, and formate. Since these salts in small quantity are substantially invisible and form invisible sulfides, they and their sulfides do not affect the optical clarity of the protective coating. A salt of zinc is preferred because zinc sulfide is white and does not tend to discolor the highlights of the image on which it is present.

Numerous water-miscible organic solvents may be employed in the coating formulation described herein. Examples are low molecular weight alcohols, such as methanol, ethanol, propanol, and isopropanol, and low molecular weight ketones, such as acetone, and methylethyl ketone.

The acid polymers which are incorporated into coating compositions within the scope of this invention are preferably cellulose or vinyl film-forming polymers having acid radicals, such as carboxylic acid (-COOH) radicals attached thereto. It is also within the scope of this invention to employ polymers with carboxylic acid anhydride groups. As examples of acid polymeric materials which are operative in this invention, mention may be made of cellulose ester derivatives, such as cellulose acetate hydrogen phthalate; partial esters of ethylene/ maleic anhydride copolymers; and partial esters of methylvinyl ether/maleic anhydride copolymers.

An additional class of acid polymers which may be used in the coating compositions of this invention are acidsubstituted partial acetals of polyvinyl alcohol composed of the recurring unit having the general formula:

[OHz-(|3HOH2C\JHCH2CH] o H OH where R is selected from the group consisting of COOH and (CH COOH and m is an integer from 1-4, inclusive. Such polymers may be prepared, for example, by reacting a low molecular weight polyvinyl alcohol, preferably one which is completely hydrolyzed, with a compound having the formula R-CHO, Where R has the same meaning as is given above, in the presence of an acid catalyst. Alternatively, the polymers may be prepared by reacting polyvinyl alcohol with an ester having the formula:

where m is an integer from l-4, inclusive, and each R is an alkyl radical, or the corresponding acetals of the above esters, and subsequently hydrolyzing the ester group to the corresponding acid. In the preferred polymeric formulations, a total of 28% to 42% of the hydroxyl groups on the polyvinyl alcohol chain are substituted with the acid function groups. It has been found that if the substitution is as low as 20%, the resulting coating composition may be undesirably viscous. The substitution can theoretically be as high as about but there are substantially no advantages to be gained thereby.

In one embodiment of this invention, the coating contains zinc oxide as the salt of a heavy metal, and specifically, ammonia as the alkali. It is believed that when so combined, the zinc oxide and ammonia react to form a zinc ammonium complex which complex, in turn, enters a state of equilibrium with Zinc oxide and ammonia. These reactions may be represented as follows:

When the composition is applied as a coating to photographic images, ammonia evaporates, generating addi tional zinc hydroxide. As zinc hydroxide is formed, how ever, it reacts with the available acid groups on the chain of the acid film-forming polymer in such a way as to cross-link the polymer. Thus, for example, where the acid film-forming polymer is a phthalaldehydic acid partial acetal of polyvinyl alcohol, it is believed that the final dried coating on the silver transfer photograph would comprise a cross-linked polymer composed of the recurring unit having the formula:

The use of acidic film-forming materials such as those disclosed above used in conjunction with zinc oxide in coating compositions for photographic silver images is disclosed and claimed in the copending application of Howard C. Haas, Ser. No. 487,596, filed concurrently herewith; the use of acid-substituted partial acetals of polyvinyl alcohol in particular for such coating compositions is disclosed and claimed in the copending application of Howard C. Haas, Ser. No. 487,597, also filed concurrently herewith.

In some cases, hydrolyzed gelatin is added to the polymeric or cellulosic material to insure the stability and the optimum coating viscosity thereof. While this addition is generally optional where the polymeric materials are used, with cellulosic esters such as cellulose acetate hydrogen phthalate, it is preferred, in order to maintain the desired physical properties. Unhydrolyzed gelatin is unsuited to the above purposes, owing to the fact that in the presence of other film-forming materials it tends to set the coating dispersion to a rigid gel. The hydrolyzed gel may be prepared by hydrolyzing either alkali or acid extracted gelatin with a proteolytic enzyme such as papain, pepsin, or trypsin, and, when a desired degree of hydrolysis is reached, by stabilizing the solution with a preservative, such as methyl p-hydroxy benzoate, cetyl pyridinium chloride, or oxyquinoline. De-

tails of such a procedure may be found in US. Pat. No. 2,794,740, issued June 4, 1957 to Edwin H. Land and Milton Green.

It is also sometimes desirable to add an additional polymeric material to the coating composition to further enhance its protective properties. The preferred material for this purpose is a hydantoin formaldehyde condensation polymer, such as that described in US. Pat. No. 2,874,045, issued Feb. 17, 1959 to Edwin H. Land, for Process of Washing and Protecting Photographic Silver Images.

Additionally, in some embodiments of the instant coating composition, it has been found advantageous to incorporate small quantities of a casein derivative, in order to provide coatings having improved characteristics. Casein derivatives suitable for this purpose are those consisting of water-dispersible casein-type proteins which form water-insoluble, transparent films on drying.

The stabilizing alkaline coating compositions as described above may be applied to photographic silver images, for example, by means of an absorbent applicator composed, for example, of a flannel, cotton batting, or cellulose sponge which is charged with the composition. When the face of the image is swabbed with such an applicator, residual reagents of the image are dissolved in the composition, and, for the most part, transferred into the applicator and the image becomes coated with a thin layer of the composition. The composition layer is then dried to form a water-insoluble protective coatmg.

Compositions within the scope of this invention may be prepared by hydrolyzing the acid film-forming material in hot water; adding the alkali and organic solvent; then adding any other desired material; and making any necessary pH adjustment. This general process results in compositions from which coatings may be applied having an especially high degree of flexibility. Alternatively, the acid film-forming material may be suspended in cold water and the alkali and organic solvent added slowly to form a solution; thereafter any additional materials are added and any necessary pH adjustment made. This latter method generally results in a composi tion forming particularly water-insoluble coatings.

The preparation of various coating compositions within the scope of this invention are illustrated by the following nonlimiting examples.

EXAMPLE I A solution was prepared by blending 30 g. of low viscosity poly-(ethylene/maleic anhydride) [commercially available from Monsanto Chemical Co., St. Louis, Mo., under the trade name DX-840-11 Resin], with 35 cc. of 30% ammonium hydroxide and 100 cc. of water.

To 40 cc. of this solution was added 5 cc. of hydrolyzed gelatin prepared by the method given above, cc. of 2-propanol, and 10 cc. of a solution prepared by dissolving 40 g. zinc acetate in 100 cc. of 30% ammonium hydroxide.

A second solution was prepared by dissolving 20 g. of modified ammonium caseinate [commercially available from the Borden Chemical Company, New York, N.Y., under the trade name Protovac PV 8979], and 30 cc. of 30% ammonium hydroxide in 100 cc. of water. cc. of this second solution was added to the initially prepared resin-gelatin solution. The dispersion formed thereby was thoroughly blended and filtered through celite; the pH of the final products was 9-10.

EXAMPLE H 40 cc. of an aqueous solution of DX-840-l1 Resin and ammonium hydroxide prepared as in Example I was blended with 10 cc. of isopropanol, 5 cc. of a 28% solution of zinc acetate in water, and 5 cc. of a solution of zinc acetate in 30% ammonium hydroxide. Thereafter, 15 cc. of hydrolyzed gelatin was added and the dispersion was thoroughly mixed. Additional 30% ammonium hydroxide was added to bring the pH to about 9-10. The final dispersion was then filtered as in Example I to provide the product.

It was noted that when the foregoing composition was applied as a photographic coating, the coating was more flexible where heat was employed to enhance dissolution of the ingredients, but at the same time, the coating became slightly more permeable to moisture and exhibited slightly less resistance to attack on the silver image by sulfur. These problems were satisfactorily overcome by the addition of a casein derivative as described in Example I.

EXAMPLE III A solution was prepared by blending 17 g. of butyl half ester of medium viscosity poly-(methyl vinyl ether/ maleic anhydride) [commercially available from General Aniline & Film Corporation, New York, N.Y., under the trade name Gantrez AN119] with 30 c. of 30% ammonium hydroxide and cc. of water.

To -40 cc. of this solution was added 10 cc. of hydrolyzed gelatin prepared as described above, 10 cc. of a solution prepared by dissolving 20 g. of zinc acetate in 100 cc. of 30% ammonium hydroxide. The final pH was 9-10.

EXAMPLE IV A first solution was prepared by blending 30 g. of DX-840-l1 Resin, 10 g. of high viscosity poly-(ethylene/ maleic anhydride) [commercially available from Monsanto Chemical Co., St. Louis, Mo., under the trade name DX84031 Resin], 60 cc. of 30% ammonium hydroxide, and 200 cc. of water. A second solution was prepared by blending 15 g. of Gantrez AN-119 Resin, 20 cc. of 30% ammonium hydroxide, and 100 cc. of water.

50 cc. of each of the above solutions were thoroughly mixed together to provide the final composition.

EXAMPLE V Acomposition was prepared which comprised 4 g. cellulose acetate hydrogen phthalate, 12 cc. of isopropyl alcohol, 4 cc. of 30% ammonium hydroxide solution, and 20 cc. of water. The mixture was allowed to stand overnight, and 10 cc. of water having dissolved therein 2 g. of zinc acetate was added. (It was found that if the zinc acetate was added immediately, it tended to precipitate; if, on the other hand, it was added after the dispersion had been allowed to stand for a short period of time, the resulting mixture was entirely stable.) The final pH was 9-10.

EXAMPLE VI A composition similar to that described in Example V was prepared, except that 40 cc. of hydrolyzed gelatin was added to 40 cc. of the cellulose acetate hydrogen phthalate solution prior to the addition of Zinc acetate.

When cellulose esters such as cellulose acetate hydrogen phthalate are used in alkaline solution, there is some tendency toward hydrolysis of the ester during storage as a solution over extended periods of time. Hence, although coatings so prepared exhibit excellent stabilizing characteristics when relatively fresh, if the composition is to be stored for extended periods prior to use, this tendency must be taken into consideration.

EXAMPLE VII A solution was prepared by blending 15 g. of DX-840- 11 Resin with 100 cc, of water and sufficient ammonia to completely dissolve the resin. To 40 cc. of this solution was added 15 cc. of hydrolyzed gelatin, 10 cc. of isopropanol, and 10 cc. of 30% aqueous ammonia having dissolved therein 2 g. of zinc acetate. Finally 2 cc. of a 75% aqueous solution of dimethyl hydantoin formaldehyde condensation polymer was added, to provide the final composition. The pH of the composition was about 9.

7 EXAMPLE vnr A phthalaldehydic acid partial acetal of polyvinyl alcohol was prepared as follows: 88 g. (2 moles) of low viscosity, low molecular weight polyvinyl alcohol having less than 1% acetate were dissolved in 500 cc. of water and cooled. 150 g. (1 mole) of phthalaldehydic acid were added with stirring and the mixture was heated at 50 C. under nitrogen. 1 cc. of concentrated sulfuric acid was added. The polymer separated as a soft, rubbery mass. After about four hours, the liquid was decanted and the product washed in hot water. It was further purified by precipitation from ethanol-water (containing 1 g. of potassium acetate) into water, washed in water and dried. The final product was predominantly the phthalaldehydic acid partial acetal of polyvinyl alcohol, with a small amount of the corresponding phthalide; the composition was confirmed by infrared analysis.

A composition for washing and protecting photographic silver images was prepared by combining and blending the following materials to provide a uniform solution:

Phthalaldehydic acid partial acetal of polyvinyl alcohol10 g.

Dimethyl hydantoin formaldehyde condensation polymer-13.5 g.

Zinc acetate3.24 g.

Water-50 cc.

Isopropyl alcohol-30 cc.

Ammonia (29.4% aqueous solution)-l cc.

The final pH was about 9.

EXAMPLE IX A composition for washing and protecting photographic silver images was prepared by combining and blending the following materials to provide a uniform solution:

Phthalaldehydic acid partial acetal of polyvinyl alcohol,

prepared as in Example VIII- g.

Dimethyl hydantoin formaldehyde condensation polymer-43.5 g.

Zinc oxide1.44 g.

Water50 cc.

Isopropyl alcohol30 cc.

Ammonia (29.4% aqueous solution)-10 cc.

The final pH was about 9.

Photographic silver transfer images were prepared by exposing photosensitive silver halide elements to a stepwedge, and developing the resulting latent images by spreading between the exposed photosensitive elements and superposed image-receiving elements a processing composition containing a viscous aqueous solution of a silver halide developing agent, a silver halide solvent, and an alkali. The respective elements were maintained in superposed relation for a predetermined period, during which time exposed silver halide was reduced to silver in the photosensitive elements. The unrednced silver halide formed a water-soluble complex silver salt which diffused through the layer of composition to the image-receiving elements where, upon being reduced to silver, it formed a silver print. At the end of the predetermined period, the photosensitive elements, together with the layer of composition, were stripped from the image-receptive elements.

The photographic silver images so prepared were swabbed by means of an absorbent applicator with the solutions of Examples I through IX. The photographic prints so treated dried rapidly under ordinary atmospheric conditions, after which they were subjected to the following tests:

(a) The prints were allowed to remain in closed chambers at a temperature of 100 F. and 80% relative humidity for a period of about 40 hours.

(b) The prints were suspended over water at 100 F. for a period of about ten days.

(c) The prints were subjected to dry air at 100 F. and 20% relative humidity for a period of about 16 hours, to test for cracking of print layer.

((1) The prints were subjected to 4 and 8 hour periods in an atmosphere containing hydrogen sulfide (provided by an aqueous solution of sodium sulfide) at room temperature.

(e) The prints were stacked face-to-face at relative humidities of 20% and a weight was placed on the stack.

After all of the foregoing tests, it was observed that in none of the prints was there any deterioration of the silver image, nor was there any loss in the density of the light steps of the image. The prints were, in general, nontacky, as evidenced by their failure to adhere to one another in test (e) above. Moreover, there was little or no image discoloration in any of the prints.

With regard to the compositions described herein, as was stated above, it is essential that the materials be alkaline. Although precise pH control is not necessary, it has been found that where the compositions were of a pH of less than about 9, the final coatings on the photographic images sometimes exhibited streaking and unevenness, even when carefully applied. Accordingly, the preferred pH range of the compositions within the scope of this invention is from about 9-14.

Since certain changes may be made in the above processes and products Without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be inter preted as illustrative and not in a limiting sense.

What is claimed is:

1. In a process for washing and protecting a diffusion transfer photographic silver print containing a residue of the reagents with which it has been processed, the steps of applying to the face thereof a solution comprising a film-forming material selected from the group consisting of cellulose and vinyl polymers containing acid groups selected from carboxylic acid and carboxylic anhydride groups, and a sufficient quantity of a volatile base to establish a pH in said solution of at least 9; and removing said volatile base by drying said layer.

2. The process of claim 1 wherein said volatile base is ammonia.

3. In a process for washing and protecting a diffusion transfer photographic silver print containing a residue of the reagents with which it has been processed, the steps of applying to the face thereof, in a thin layer, a solution comprising:

(1) a film-forming polymeric material selected from the group consisting of cellulose and vinyl polymers containing acid groups selected from carboxylic acid and carboxylic acid anhydride groups,

(2) water,

(3) a water-miscible organic liquid that is a solvent for said film-forming polymeric material,

(4) a salt of a heavy metal which forms an insoluble sulfide, and

(5) a volatile base in sufficient quantity to establish a pH in said solution of between 9 and 14; and removing said volatile base by drying said layer.

4. The process of claim 3 wherein said organic liquid is a low molecular Weight alcohol.

5. The process of claim 3 wherein said organic liquid is a low molecular weight ketone.

6. The process of claim 3 wherein said volatile base is ammonia.

7. The process of claim 3 wherein said salt of a heavy metal is a zinc salt.

8. The process of claim 3 wherein said salt of a heavy metal is zinc acetate.

9. The process of claim 3 wherein said cellulose polymer is a cellulose ester.

10. The process of claim 3 wherein said cellulose polymer is cellulose acetate hydrogen phthalate.

11. The process of claim 3 wherein said vinyl polymer is poly(ethylene/rna1eic anhydride).

12. The process of claim 3 wherein said vinyl polymer contains carboxylic anhydride groups.

References Cited UNITED STATES PATENTS Land et a1. 9629 Land et a1 9650 Land 9629 10 Land et a1 9629 Land 9629 Land et a1. 9629 10 2,979,477 4/1961 Land 260-294 3,174,858 3/1965 Van Hoof et a1. 9629 3,325,283 '6/ 1967 Barstow et a1. 96-3 FOREIGN PATENTS 496,049 11/ 1938 Great Britain.

NORMAN G. TORCHIN, Primary Examiner M. F. KELLEY, Assistant Examiner U.S. C1. X.R. 9650 

